A phase-locked oscillator is compact and has low phase noise and low spurious signal output, and because of these characteristics it is often used as a local oscillator.
In an apparatus in which a phase-locked oscillator is employed as a local oscillator for frequency conversion, it is generally so designed that, if operation is out of phase, the apparatus stops functioning. For this purpose, it is essential to accurately determine whether operation is out of phase or not.
Therefore, a conventional phase-locked oscillator has an arrangement as illustrated in FIG. 7.
That is, an input signal applied to a reference signal input terminal 9 is compared in phase with an output of a voltage-controlled oscillator 4 by a phase comparator 1. An output of the phase comparator 1 is input to a loop filter 2. An output from the loop filter 2 is branched into two, one of which is input to the voltage-controlled oscillator 4, while the other is supplied to a sweep oscillator 3. An output from the sweep oscillator 3 is input to the loop filter 2. An output from the voltage-controlled oscillator 4 is branched into three so that they are respectively input to a band-pass filter 17, the phase comparator 1, and an oscillation output terminal 11. An output of the band-pass filter 17 is input to an amplitude detector 18, and an output of the amplitude detector 18 is input to a level detector 19. In turn, a phase lock detection signal is derived from a phase lock detection signal terminal 10 of the level detector 19. Thus, if the frequency of an oscillation output signal happens to deviate from a predetermined value, the oscillation output signal cannot pass through the band-pass filter 17, and accordingly the level of the output from the amplitude detector 18 is lowered. The fall in the output level is detected by the level detector 19, and on the basis of the detection an output signal for stopping the operation of the apparatus is issued from the phase lock detection signal terminal 10.
In such an arrangement, however, there has been a problem that, even if an out-of-phase phenomenon occurs, this cannot be detected in some cases. More particularly, when the output frequency of the voltage-controlled oscillator 4 in synchronous or phase-locked state deviates by a small degree from that in out-of-phase state, it is difficult to judge the occurrence of the out-of-phase state, since the frequency of the output signal from the voltage-controlled oscillator 4 is within the band of the band-pass filter 17 even when it is out of phase. In order to overcome such difficultly, it may be conceivable to narrow the band of the band-pass filter 17; but an attempt to narrow the band in a high-frequency zone has its limit, resulting in that this problem remains to be solved.